Spatial effects, sampling errors, and task specialization in the honey bee

Experimental increase of worker diversity benefits brood production in ants

BACKGROUND

The reproductive division of labor of eusocial insects, whereby one or several queens monopolize reproduction, evolved in a context of high genetic relatedness. However, many extant eusocial species have developed strategies that decrease genetic relatedness in their colonies, suggesting some benefits of the increased diversity. Multiple studies support this hypothesis by showing positive correlations between genetic diversity and colony fitness, as well as finding effects of experimental manipulations of diversity on colony performance. However, alternative explanations could account for most of these reports, and the benefits of diversity on performance in eusocial insects still await validation. In this study, we experimentally increased worker diversity in small colonies of the ant Lasius niger while controlling for typical confounding factors.

RESULTS

We found that experimental colonies composed of workers coming from three different source colonies produced more larvae and showed more variation in size compared to groups of workers coming from a single colony.

CONCLUSIONS

We propose that the benefits of increased diversity stemmed from an improved division of labor. Our study confirms that worker diversity enhances colony performance, thus providing a possible explanation for the evolution of multiply mated queens and multiple-queen colonies in many species of eusocial insects.

Increased immunocompetence and network centrality of allogroomer workers suggest a link between individual and social immunity in honeybees

The significant risk of disease transmission has selected for effective immune-defense strategies in insect societies. Division of labour, with individuals specialized in immunity-related tasks, strongly contributes to prevent the spread of diseases. A trade-off, however, may exist between phenotypic specialization to increase task efficiency and maintenance of plasticity to cope with variable colony demands. We investigated the extent of phenotypic specialization associated with a specific task by using allogrooming in the honeybee, Apis mellifera, where worker behaviour might lower ectoparasites load. We adopted an integrated approach to characterize the behavioural and physiological phenotype of allogroomers, by analyzing their behavior (both at individual and social network level), their immunocompetence (bacterial clearance tests) and their chemosensory specialization (proteomics of olfactory organs). We found that allogroomers have higher immune capacity compared to control bees, while they do not differ in chemosensory proteomic profiles. Behaviourally, they do not show differences in the tasks performed (other than allogrooming), while they clearly differ in connectivity within the colonial social network, having a higher centrality than control bees. This demonstrates the presence of an immune-specific physiological and social behavioural specialization in individuals involved in a social immunity related task, thus linking individual to social immunity, and it shows how phenotypes may be specialized in the task performed while maintaining an overall plasticity.

A computational model of task allocation in social insects: ecology and interactions alone can drive specialisation

Social insects allocate their workforce in a decentralised fashion, addressing multiple tasks and responding effectively to environmental changes. This process is fundamental to their ecological success, but the mechanisms behind it are not well understood. While most models focus on internal and individual factors, empirical evidence highlights the importance of ecology and social interactions. To address this gap, we propose a game theoretical model of task allocation. Our main findings are twofold: Firstly, the specialisation emerging from self-organised task allocation can be largely determined by the ecology. Weakly specialised colonies in which all individuals perform more than one task emerge when foraging is cheap; in contrast, harsher environments with high foraging costs lead to strong specialisation in which each individual fully engages in a single task. Secondly, social interactions lead to important differences in dynamic environments. Colonies whose individuals rely on their own experience are predicted to be more flexible when dealing with change than colonies relying on social information. We also find that, counter to intuition, strongly specialised colonies may perform suboptimally, whereas the group performance of weakly specialised colonies approaches optimality. Our simulation results fully agree with the predictions of the mathematical model for the regions where the latter is analytically tractable. Our results are useful in framing relevant and important empirical questions, where ecology and interactions are key elements of hypotheses and predictions.

Honey bee colony performance and health are enhanced by apiary proximity to US Conservation Reserve Program (CRP) lands

Honey bee colony performance and health are intimately linked to the foraging environment. Recent evidence suggests that the US Conservation Reserve Program (CRP) has a positive impact on environmental suitability for supporting honey bee apiaries. However, relatively little is known about the influence of habitat conservation efforts on honey bee colony health. Identifying specific factors that influence bee health at the colony level incorporates longitudinal monitoring of physiology across diverse environments. Using a pooled-sampling method to overcome individual variation, we monitored colony-level molecular biomarkers during critical pre- and post-winter time points. Major categories of colony health (nutrition, oxidative stress resistance, and immunity) were impacted by apiary site. In general, apiaries within foraging distance of CRP lands showed improved performance and higher gene expression of vitellogenin (vg), a nutritionally regulated protein with central storage and regulatory functions. Mirroring vg levels, gene transcripts encoding antioxidant enzymes and immune-related proteins were typically higher in colonies exposed to CRP environments. Our study highlights the potential of CRP lands to improve pollinator health and the utility of colony-level molecular diagnostics to assess environmental suitability for honey bees.

Honey bees overwintering in a southern climate: longitudinal effects of nutrition and queen age on colony-level molecular physiology and performance

Honey bee colony nutritional ecology relies on the acquisition and assimilation of floral resources across a landscape with changing forage conditions. Here, we examined the impact of nutrition and queen age on colony health across extended periods of reduced forage in a southern climate. We measured conventional hive metrics as well as colony-level gene expression of eight immune-related genes and three recently identified homologs of vitellogenin (vg), a storage glycolipoprotein central to colony nutritional state, immunity, oxidative stress resistance and life span regulation. Across three apiary sites, concurrent longitudinal changes in colony-level gene expression and nutritional state reflected the production of diutinus (winter) bees physiologically altered for long-term nutrient storage. Brood production by young queens was significantly greater than that of old queens, and was augmented by feeding colonies supplemental pollen. Expression analyses of recently identified vg homologs (vg-like-A, -B, and -C) revealed distinct patterns that correlated with colony performance, phenology, and immune-related gene transcript levels. Our findings provide new insights into dynamics underlying managed colony performance on a large scale. Colony-level, molecular physiological profiling is a promising approach to effectively identify factors influencing honey bee health in future landscape and nutrition studies.

Evolution of self-organized division of labor in a response threshold model

Division of labor in social insects is determinant to their ecological success. Recent models emphasize that division of labor is an emergent property of the interactions among nestmates obeying to simple behavioral rules. However, the role of evolution in shaping these rules has been largely neglected. Here, we investigate a model that integrates the perspectives of self-organization and evolution. Our point of departure is the response threshold model, where we allow thresholds to evolve. We ask whether the thresholds will evolve to a state where division of labor emerges in a form that fits the needs of the colony. We find that division of labor can indeed evolve through the evolutionary branching of thresholds, leading to workers that differ in their tendency to take on a given task. However, the conditions under which division of labor evolves depend on the strength of selection on the two fitness components considered: amount of work performed and on worker distribution over tasks. When selection is strongest on the amount of work performed, division of labor evolves if switching tasks is costly. When selection is strongest on worker distribution, division of labor is less likely to evolve. Furthermore, we show that a biased distribution (like 3:1) of workers over tasks is not easily achievable by a threshold mechanism, even under strong selection. Contrary to expectation, multiple matings of colony foundresses impede the evolution of specialization. Overall, our model sheds light on the importance of considering the interaction between specific mechanisms and ecological requirements to better understand the evolutionary scenarios that lead to division of labor in complex systems.